Materials such as metal can creep if they experience stress at a high enough temperature, over a long period of time. One of the primary aims of a designer of equipment operating in a creep range is to plan for the possibilities of failure by excessive deformation and avoid such failure. For example, dimensional changes with time, even of the order of a few millimetres, leading to mismatch of components or contact between moving parts, could lead to serious damage to plant, not to mention loss of human life and revenue.
Therefore there is a need for the generation of ways of predicting whether components operating in the creep range will sustain the life required of them. The useful life of a material could be terminated when deformation becomes excessive, when rupture occurs or when latent flaws or initiated cracks grow at unacceptable rates by creep or creep/fatigue.
Creep is defined as a high temperature progressive deformation at constant stress. “High temperature” is a relative term dependent upon the materials involved. Creep rates are used in evaluating materials for boilers, gas turbines, jet engines, ovens, or any application that involves high temperatures under load. Understanding high temperature behaviour of metals is useful in designing failure resistant systems.
Conventional uniaxial creep tests using cylindrical bar type specimens are usually performed for determining the creep properties. Creep occurs in three stages: Primary, or Stage I; Secondary, or Stage II: and Tertiary, or Stage III. Stage I, or Primary creep occurs at the beginning of a components life during which the strain rate reduces. Resistance to creep increases until Stage II is reached. In Stage II, or Secondary creep, the rate of creep becomes roughly steady. This stage is often referred to as steady state creep. In Stage III, or tertiary creep, the creep rate begins to accelerate as the cross sectional area of the specimen decreases due to (i) material degradation, and (ii) necking or internal voiding which decreases the effective area of the specimen. If stage III is allowed to proceed, fracture will occur.
Creep tests are usually employed to determine the minimum creep rate in Stage II. Engineers need to account for this expected deformation when designing systems. Currently there are three types of tests commonly used to determine creep strain data for a sample of material which is relatively small in size. These are the uniaxial, impression and small punch tests which are described further below.
To determine creep properties from the uniaxial creep test using small uniaxial specimen, material is subjected to prolonged constant tension or compression loading at constant temperature. Deformation is recorded at specified time intervals and a creep strain vs. time diagram is plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates the test and Time for Rupture is recorded.
Impression creep testing, wherein a flat-ended cylindrical punch is used to load a small area of the specimen in compression, enables testing of small material volumes and the use of miniaturized specimens, with minimal preparation. However, under very slow creeping situations (depending on material, stress and temperature), a considerable time is necessary for the evolution of a steady-state creep zone under the punch, resulting in a prolonged period of decreasing creep rate even when the constitutive creep behaviour is strain-independent. This test-dependent transient behaviour, where the creep rate decreases slowly even at very long test times, complicates the determination of true material creep parameters. The very small creep deformations which occur under such conditions can be very difficult to accurately measure.
Small punch creep (SPC) test technique is another method used to evaluate the creep properties of materials extracted from components in a high temperature environment. The SPC uses a number of theory and test techniques for the purpose of applying the SPC test to the residual creep life assessment of plant components. However, as it is carried out in a high temperature environment using very thin miniature disc specimens, which is different from conventional uniaxial creep tests described above. The creep properties of the test specimens can be significantly influenced by surface oxidation. Moreover, when creep and oxidation occur simultaneously, it becomes difficult to unambiguously distinguish which changes are attributable to oxidation and which to fracture. When the oxide is strong and firmly adherent to the metal, it can improve creep strength, but if the oxide had low inherent strength or spalls away from the surface, creep strength can decrease.
The advantage that the small punch test method has relative to the impression creep test method is that it involves tertiary creep and fracture, whereas the impression creep test method only gives information on secondary creep behaviour and some indication about primary creep. However, so far the small punch test method does not have a fully verified and universal route for interpreting the primary, secondary and tertiary contributions to the overall displacement versus time curve.
For practical small specimen dimensions, the “test section” area for all three specimen types can be made large enough to enable “bulk” creep properties to be obtained (as opposed to single grains, or a few grain boundary properties being determined, which can be the case if the sample is very small). Similarly, all three have relatively small equivalent gauge length (EGLs), i.e. 2 to 10 mm for practical dimensions. The term EGL is defined as the length by which the measured deformation must be divided to give the equivalent strain to that which would be obtained from a corresponding uniaxial test.
The term “gauge length” is used in mechanical testing, it is the length, usually marked on a portion of a tensile test piece over which the elongation is measured.
In order to obtain results in reasonably short timescales (tests are usually carried out for times in the range 500 h to 3000 h), and in order to get easily measureable deformations with high accuracy, tests are often conducted at relatively high “stress” levels and/or temperatures, as compared with typical values of stress and temperature that a component would be designed to endure. All three specimen types currently being used are relatively “stiff” and so there is little scope for significantly increasing the EGL.
Any discussion of documents, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art.